The hemoglobin 293cys residue is conserved and received much attention recently as a potential modulator of how red cells affect vascular nitric oxide (NO) metabolism and function. However, the precise function and mechanisms involved remain unclear. Understanding how this residue controls vascular nitric oxide function is critical since dysfunction in these mechanisms may contribute to a number of vascular pathological states. In this proposal we build upon preliminary data generated from novel mouse models that express exclusively either wild-type human hemoglobin or human hemoglobin in which the 293cys residue has been replaced with an Ala in their red cells. Specifically, we present data indicating that under physiological conditions, deoxygenation of red cells and hemoglobin activates a nitrite reductase activity that results in the one-electron reduction of the anion nitrite to NO. This process is regulated allosterically by controlling hemoglobin oxygen affinity and interestingly modulated by the 293cys residue. In this context we propose that the 293cys residue is critical in coupling hemoglobin oxygen sensing with nitrite derived NO-bioactivity. In contrast, during the acute inflammatory disease Sepsis, we propose that the 293cys is a target for nitrosative stress forming S-nitrosohemoglobin (SNOHb), which in turn contributes to the vascular and pulmonary dysfunction associated with this disease. The latter is indicated by data showing SNO-containing red cells can stimulate neutrophil adhesion to pulmonary endothelial cells and elicit vasodilation in a manner that is independent on allosteric regulation. Both pulmonary inflammation and lung injury are features of sepsis. These novel concepts will be investigated in this proposal by pursuit of the hypothesis that during acute inflammation, the role of the 293cys residue as a modulator of vascular NO-signaling changes from a nitrite-reductase dependent to SNOHb dependent mechanism which will be tested via the following specific aims 1) Determine the mechanism by which 293cys regulates RBC dependent NO vascular cell signaling., 2) Determine the role of 293cys residue in controlling nitrite reduction and NO- dependent vascular cell signaling in vivo, 3) Determine the role of the 293cys residue in affecting RBC effects during Sepsis induced hypotension and pulmonary inflammation. Accomplishment of these aims will yield insights into the mechanisms novel therapeutic targets focusing on how RBCs modulate NO-metabolism.